The invention relates to the rotor of a small size coreless motor and a method of manufacturing same.
A coreless motor, in particular, that which is used in a small size tape recorder must maintain a constant speed of rotation. In addition, it is also necessary that the rotation be uniform, smooth and stabilized. This requires a dynamic balance of the rotor relative to the axis of rotation thereof. However, in conventional coreless motors of the kind described, a jig is used to assemble rotor coils around the rotary shaft of the motor by a manual operation, and then a molding material is cast to form the rotor. With this procedure, it is very difficult to achieve a satisfactory dynamically balanced rotor.
Before describing the present invention, an example of the prior art will be described with reference to FIGS. 1 to 5, which show a rotor including three coils. A coreless motor 1 is shown as comprising a housing 7 which includes a hollow cylinder 6 and a pair of upper and lower end plates 5a, 5b secured to the opposite ends thereof. A pair of bearings 8a, 8b are centrally fitted in the end plates 5a, 5b to receive a rotary shaft 2 in a rotatable manner. A rotor 4 including a commutator 3, and rotor coils 15a, 15b, 15c (see FIG. 5) is integrally connected with the shaft 2, the lower end of which bears against an abutment 9 secured to the lower end plate at a position beneath the bearing 8b. The upward thrust is countered by the abutment of the upper end face of the commutator 3 against the lower end face of the bearing 8a with a spacer 13 interposed therebetween. The motor also includes a magnet 10 which is secured within the housing 7 so as to be opposite to the upper face of the rotor 4, and a pair of brushes 11a, 11b which are fixedly mounted on the housing 7 for engaging the commutator 3.
In the prior art practice, the manufacturing of the rotor 4 takes place in the manner mentioned below. Initially, the commutator 3 is integrally connected with the rotary shaft 2 in concentric manner, by using an adhesive 12, as shown in FIG. 2. Then the commutator 3 is fitted into a through opening 14a formed centrally in the bottom of a saucer-shaped casting jig 14 shown in FIG. 3. As shown in FIGS. 4A and B, rotor coils 15a, 15b, 15c, which may be formed by a conductive wire or strip, have their opposite ends electrically connected with the commutator 3 as by soldering, and are then disposed as shown in FIG. 5. When the rotary shaft 2, commutator 3 and rotor coils 15a to 15c are disposed in the jig 14 in this manner, a molding material 16 such as epoxy resin is cast into the jig and is caused to set, whereupon the jig 14 is removed to form the rotor 4 complete with the rotary shaft 2 and the commutator 3, as shown in FIG. 1. To achieve a dynamically balanced rotor 3 with respect to the rotary shaft 2, it is necessary that the coils 15a to 15b be disposed at an equal spacing between them within the jib 14, but since the disposition is performed by a manual operation, a movement of the coil or coils on the jig is likely to occur, rendering the positioning operation very difficult. Even if extreme care is exercised and a substantial length of time is devoted to their alignment there is nevertheless much difficulty in obtaining the desired the dynamic and magnetic balance, preventing a smooth rotation of the motor 1.